In 1936 during the Spanish Civil War, anti-clerical Loyalists are in command of a small city where disillusioned priest Arturo Carrera confronts his Bishop. Despairing the church's "empty piety," Arturo forsakes his vows and leaves the cathedral dressed in civilian clothes. That evening, as a rebel air raid sends the city into chaos, Arturo takes shelter in a building where he meets Soledad, a cabaret girl. Meanwhile, the Bishop gives a sacred relic, the blessed blood of Saint John, to priest Canon Rota, ordering him to take the relic to the rebels, who are faithful to the church. Soon after, a Loyalist mob breaks into the cathedral, where they ransack the altars and murder several priests. That night, the Loyalists post descriptions of the two escaped priests, Arturo and Rota, through out the city. Fleeing the police, Arturo slips into a cabaret and takes a private room with Soledad. Unaware that Arturo is a priest on the run, she reveals that she cares little for priests, but does not believe people should harm the church. When soldiers enter the cabaret in search of Arturo, the brave Soledad, realizing his real identity, takes him to her apartment, where Arturo admits that "his faith has failed him." Despite his protests that she not become involved in the danger, Soledad insists she has had a premonition that Arturo is special and must be protected. When soldiers suddenly knock on the outside door, Arturo surrenders to avoid implicating Soledad. Within days, the frantic parishioners have covered the city in graffitti asking for the location of the relic. After cynical Communist Gen. Clave arrives in town, American correspondent Hawthorne, who is writing a story about the relic, explains the local legend. Whoever has possession of the relic is said to be impossible to defeat. Despite Clave's own indifference, he orders his soldiers to find the relic to help instill confidence in the Loyalist cause among the townspeople. Although Arturo is unable to provide information about the location of the relic, Capt. Botargas accepts his offer to work for the government and help the Catholic soldiers reconcile their faith with the Loyalist's socialist agenda. After being set free, Arturo seeks consolation, friendship and finally romance with Soledad. One day, after Arturo explains to the soldiers at a public square that the relic cannot bring them victory, Rota, disguised in civilian clothes, tells him he is hiding in the church and begs for help, but Arturo refuses. Later, Soledad announces that she has quit the cabaret and professes her love to Arturo. With equal affection, Arturo explains his feelings are like that of a young schoolboy with his first love, an innocent love. That night Loyalist reinforcements arrive, but are many men short because of desertions caused by fear of the relic being in enemy hands. Meanwhile, Soledad, having learned of Rota's plea from Arturo, goes to the church where soldiers capture her and Rota. Botargas interrogates Soledad about her involvement with Rota and Arturo, threatening torture if she does not comply. Meanwhile Rota, sentenced to execution as a traitor, asks for Arturo to hear his confession. During their meeting, Rota admits that he, like Arturo, did not believe in the relic but after weeks of walking among the people, realizes that it is the people's faith in the relic that gives them hope in God. He then tells Arturo that the priests were all tortured and then killed for refusing to give information about the relic. Moved by Rota's speech, Arturo asks for the relic's location. Arturo finds the relic, but is immediately arrested outside the church. Unknown to his captors, Arturo maintains possession of the relic by hiding it in his clothing. The next morning, Clave sends over two hundred prisoners, including Arturo and Soledad, under armed guard to be a decoy for the advancing enemy. During a break in the march, Arturo tells Soledad he has the relic. That night, as they camp near Soledad's village, they realize that they will face the rebel forces the next morning. While the prisoners pray, Soledad notices Arturo is torn between his love for her and his responsibility as a priest to ease the pain of those suffering around him. Botargas calls for Soledad and reminds her that the priest will reject her to return to his profession. He then offers her freedom in trade for information about the relic. Later, after Soledad finds Arturo accepting the villagers’ confessions, she agrees to Botargas' terms. That night, Soledad admits her arrangement with Botargas to Arturo, but suggests she give Botargas a false location and then take the relic away herself. Arturo agrees to the plan, but when Soledad tells Botargas the false location, he refuses to set her free. In a surprise attack, the rebels begin shooting indiscriminately among the group, wounding Soledad, who then stumbles along the path to her village clutching the relic. When the rebels call into the church where the prisoners are held, Arturo explains to a rebel officer that they are prisoners of the Loyalists. To prove their fidelity to the church, Arturo claims to have had possession of the relic. The officer in charge replies that unless Arturo can produce the relic, he will have to execute all of them after a priest hears their confessions. Meanwhile, a rebel soldier finds Soledad collapsed holding the relic. The following morning, as Arturo prepares the prisoner to meet their fate, he suddenly hears Soledad weakly calling his name and walks out to find that a soldier has brought both Soledad and the relic to the church. After a delirious Soledad, believing she has lost the relic, asks for his forgiveness and then dies, the young officer frees the prisoners. While Soledad is returned to her village in a coffin, Arturo kisses the holy relic and places it in a sacred monument.